[Coverage][C++] VarDecl on Cond is ignored from mapping.

[chapuni]

bool foo(bool a) {
    bool x;
    if ((x = (a ? false : true))) // YOI
        return !x;
    if (bool y = (a ? true : false)) // DAME
        return y;
    return a;
}

(https://godbolt.org/z/Tch3cP4G8)

"YOI" is expected.

  Branch,File 0, 3:9 -> 3:33 = #1, (#0 - #1)
  Branch,File 0, 3:15 -> 3:16 = #2, (#0 - #2)

"DAME" doesn't emit CondOp but has only Branch for IfStmt.

  Branch,File 0, 5:14 -> 5:15 = #3, ((#0 - #1) - #3)

Looks like CoverageMappingGen.cpp doesn't traverse VarDecl on Cond.